Understanding the mechanisms involved in the induction and maintenance of cancer stem cells (CSCs) is critical for developing therapies that can target CSCs effectively. Increasing evidence has suggested such mechanisms are often shared by both CSCs and normal stem cells. Therefore, studies in normal stem cells yield important insights into the cancer stem cell biology. The epithelial-mesenchymal transition has been shown to confer differentiated mammary epithelial cells stem cell-like properties (Mani et al. Cell 2008). However, it is not clear whether the EMT plays a role in bona fide mammary stem cell induction and maintenance. It is also not yet rigorously demonstrated that the EMT can induce breast cancer stem cells. In addition, the mechanism by which the EMT activates stem-cell program is unknown. To examine the role of the EMT in the induction of stem cells, primary mammary epithelial cells (MECs) and cancer cells will be induced to undergo transient EMTs, and then tested for increases in stem cell activities. The ability of EMTs in converting differentiated cells into stem cells will also be tested. Loss-of-function approaches, including gene knock-out and knock-down, will be used to test the requirement of EMT- inducing factors in the maintenance of mammary stem cells and breast cancer stem cells. The involvement of stem cell-related signaling pathways in mediating the activation of the stem-cell program by the EMT will be also examined in this proposal. Positive outcomes ofthese studies will lead to better understanding ofthe biology of normal stem cells and cancer stem cells and help developing cancer stem cell-targeted therapies. In addition, understanding the link of EMT and bona fide stem cells will help generating normal epithelial stem cells for research and therapeutic uses.